The present invention relates to the field of mission management systems of uninhabited air vehicles, and more particularly to a method and system for automatic flight envelope protection to reduce damage and mishap rates of such vehicles.
Flight envelope protection is an extension of an aircraft guidance system that prevents the aircraft from exceeding its designed operating limits while in one of the aircraft""s standard guidance modes. Modem flight envelope protection features offer automated or semi-automated safety features. In manned aircraft, flight envelope protection is accomplished by the pilot, together with automated systems, using visual, auditory, tactile warning aids, and pilot controls.
On-ground sensing, usually provided by squat switches, is used to prevent inadvertent activation of ground spoilers and engine reverse thrust. Stall protection systems are used to warn of stalls and prevent stalls from happening. Digital engine controls provide engine protections such as thrust controls and reverse thrust controls.
These features are designed for the protection of the vehicle and its passengers and are used to prevent the vehicle from exceeding its structural and aerodynamic limitations. Exceeding a vehicle""s limitations may lead to damage or complete destruction of the vehicle, termed a mishap.
Hard protections prevent the pilot from exceeding the flight envelope. Soft protection systems incorporate soft limits that warn pilots of pending or actual envelope exceedance, but allow the pilot to override the limitations.
With uninhabited (unmanned) aircraft, protection must be achieved automatically without intervention from a ground controller or other outside agent. Existing flight envelope protection systems for manned aircraft alert the pilot, who then must assess the situation and determine what action to take. In the uninhabited aircraft, to achieve automatic flight envelope protection, the method and system must not only be capable of detecting the conditions for alerting, but also determine appropriate corrective actions. This requires sensor signal logic (to replace the pilot""s biosensory cues), and control logic, which are capable of overriding other commands (e.g., waypoint navigation) to the aircraft guidance or navigation systems whenever the aircraft enters a guidance condition which is not maintainable.
Automatic flight envelope protection may be realized as additional software on an existing guidance and navigation processor, or as a separate mission management processor and/or additional sensors and mechanical actuation devices for the aircraft. The main benefit of such protection is to reduce damage and mishap rates of unmanned vehicles by providing designed-in prevention of unsafe or unstable speeds and attitudes from which the vehicle cannot recover.
Some manned aircraft have extensions of the navigation system for route planning purposes, or have the mission plan or flight plan data entered and stored before initiation of a mission. However, manned aircraft do not usually have onboard planning. Uninhabited aircraft either have groundbased control stations (which, among other things, might transmit preplanned mission data), or mission planning systems (normally on board) which control the execution of the aircraft""s mission.
Presently available mission planning systems for generating guidance or navigation commands for a vehicle (whether it is a ground-based controller, preprogrammed operating sequence, or mission management system) may at times generate commands which the vehicle cannot safely execute. These subsystems are designed prior to flight, and typically use simplified, fleet-wide models of aircraft performancexe2x80x94hence they will occasionally issue commands that a particular vehicle cannot execute.
For efficiency reasons, users implement fleet-wide envelope protection protocols. These govern all types of vehicles within a fleet. However, certain vehicles require different commands to remain within their flight envelopes. Each vehicle type usually has its own flight envelope. In addition, under various unanticipated flight conditions, mission planning systems may produce completely erroneous outputs for short periods of time. If these commands are simply executed by the guidance, navigation, or control systems they often result in damage to the vehicle or a mishap.
Some proposed flight envelope protection systems utilize neural networks that have been trained from large sets of known input values. However, these systems cannot detect or respond properly to extreme flight conditions where neither simulated nor measured flight data are likely to be accurate.
Existing methods of flight envelope protection, which involve the addition or modification of logic hardware in the mission management, guidance and navigation systems, avionics, or primary flight control systems, are not yet fully developed for uninhabited air vehicles, are not sufficiently adaptable to conditions of actual flight, and do not incorporate an integrated systems approach that is capable of balancing vehicle safety with mission objectives under all circumstances. The prior art suffers from:
limitations and faults of the mission management system and the software which supports it;
lack of a consistent definition of the flight envelope;
not linking the flight envelope parameterization to the guidance modes of the system;
not detecting when the flight envelope is about to be exceeded;
inadequately defining a corrective action, due to dependence on the availability of input from a pilot; and,
poor integration with existing mission management, guidance navigation, and flight control systems.
Automatic Flight Envelope Protection (AFEP) is embodied in software and computational hardware which augments existing guidance, navigation and/or control systems of Uninhabited Air Vehicles (UAV""s) (also termed Unmanned, or Autonomous Air Vehicles). These include but are not limited to Cruise missiles, Unmanned Reconnaissance Vehicles (URV""s), or Remotely Piloted Vehicles (RPV""s). The invention automatically prevents UAV""s from flying outside of their safe operating limits when they are subject to guidance commands generated by on-board mission planning systems or ground-based control systems.
The embodiments of the invention involve means of defining the flight envelope that are specifically suited to UAV""S, means of anticipating and detecting actual or expected exceedance of the flight envelope and means of generating corrective actions which maintain the vehicle within the envelope, while maintaining vehicle controllability and awareness of UAV mission objectives. In addition, this invention includes new means of integrating these features into a system which is interoperable with conventional aircraft guidance, navigation, control, propulsion, and avionic subsystems.
The invention is a novel mission execution system that is capable of dynamically switching between levels: mission planner/ navigation/ guidance/ flight control. The approach is xe2x80x9cmemory-lessxe2x80x9d in that it detects and corrects problems with immediate operating conditions and guidance commands, but relies on existing xe2x80x9cmemoryxe2x80x9d of the status of other subsystems (guidance, flight control) to store state information about the system.
The flight envelope protection algorithms are the lowest level of the vehicle management system, and their corrective actions are inserted between the navigation system 20 and the guidance or flight control system 30. When the current operating point of the aircraft approaches the flight envelope too closely, or from the wrong direction, corrective actions will override or modify the navigation system inputs to the guidance system or the normal guidance system 20 inputs to the primary flight control system 30. The invention retains intact the primary flight control functions, which are usually already designed for safety in the event of equipment failures (but not for other types of mishap prevention).
The AFEP algorithms accept inputs from air data to determine the current operating point within the flight envelope, from the guidance system 20 primarily the guidance mode and heading command, and from the mission planning system 40 to determine corrective actions which are most compatible with mission parameters, viz., tactical objectives. The flight envelope and current operating point are updated dynamically based on current air and vehicle data.
The algorithms dynamically determine the most critical distance of the current operating point from the boundary of the flight envelope, and the normal (approach) component of speed, and then compute corrective actions consistent with these parameters. This is preferably done by a xe2x80x9cbounding polytopexe2x80x9d method.
The algorithms use the xe2x80x9cvirtual actuatorxe2x80x9d (Aerodynamic Control Effector, or ACE) concept to allocate force among available physical actuators. The invention also uses aerodynamic control effectors to simplify the choice of actuators to implement control actions. The advantage of this approach is that ACE systems automatically adopt effective actuator forces and operative limits in the event of equipment failure or battle damage, so that no additional changes are required in the AFEP system. In the event that an ACE system is not available, the AFEP system can be designed to operate with a fixed set of physical actuators.
The AFEP algorithm contains hybrid logic that selects the corrective action based on the guidance mode of the aircraft, if any.
AFEP is intended to reduce damage and mishap rates of such vehicles, particularly when they are subject to guidance commands that would otherwise cause the vehicle to become uncontrollable and/or to exceed other operating limits which would cause permanent damage or destruction to the vehicle or its components. The AFEP concept is typically executed onboard the aircraft rather than on the ground. A concomitant benefit is the improvement in mission success rates and UAV availability, since mission success normally requires vehicles to remain operational. Another benefit is to improve the safety of other (manned or unmanned) vehicles which must interoperate with UAV""s in commercial or military airspace.